Power Supply System (TN-C, TN-S, TN-C-S, TT, IT)
The basic power supply system used in the power supply for construction projects is three-phase three-wire and three-phase four-wire system etc, but the connotation of these terms is not very strict. The International Electrotechnical Commission (IEC) has made uniform provisions for this, and it is called TT system, TN system, and IT system. Which TN system is divided into TN-C, TN-S, TN-C-S system. The following is a brief introduction to various power supply systems.
power supply system
According to the various protection methods and terminologies defined by IEC, low-voltage power distribution systems are divided into three types according to the different grounding methods, namely TT, TN, and IT systems, and are described as follows.
TN-C power supply system
The TN-C mode power supply system uses the working neutral line as the zero-crossing protection line, which can be called the protection neutral line and can be represented by PEN.
TN-C-S power supply system
For the temporary power supply of the TN-CS system, if the front part is powered by the TN-C method, and the construction code specifies that the construction site must use the TN-S power supply system, the total distribution box can be divided at the rear part of the system. Out of the PE line, the features of the TN-CS system are as follows.
1) Working zero line N is connected with the special protection line PE. When the unbalanced current of the line is large, the zero protection of the electrical equipment is affected by the zero line potential. The TN-C-S system can reduce the voltage of the motor housing to the ground, but it cannot completely eliminate this voltage. The magnitude of this voltage depends on the load imbalance of the wiring and the length of this line. The more unbalanced the load and the longer the wiring, the greater the voltage offset of the device housing to ground. Therefore, it is required that the load unbalance current should not be too large, and that the PE line should be grounded repeatedly.
2) The PE line cannot enter the leakage protector under any circumstances, because the leakage protector at the end of the line will cause the front leakage protector to trip and cause a large-scale power failure.
3) In addition to the PE line must be connected to the N line in the general box, the N line and the PE line must not be connected in other compartments. No switches and fuses shall be installed on the PE line, and no earth shall be used as the PE. line.
Through the above analysis, the TN-C-S power supply system is temporarily modified on the TN-C system. When the three-phase power transformer is in good working ground condition and the three-phase load is relatively balanced, the effect of the TN-C-S system in construction electricity use is still feasible. However, in the case of unbalanced three-phase loads and a dedicated power transformer on the construction site, the TN-S power supply system must be used.
TN-S power supply system
The TN-S mode power supply system is a power supply system that strictly separates the working neutral N from the dedicated protection line PE. It is called the TN-S power supply system. The characteristics of the TN-S power supply system are as follows.
1) When the system is running normally, there is no current on the dedicated protection line, but there is unbalanced current on the working zero line. There is no voltage on the PE line to the ground, so the zero protection of the metal shell of the electrical equipment is connected to the special protection line PE, which is safe and reliable.
2) The working neutral line is only used as a single-phase lighting load circuit.
3) The special protection line PE is not allowed to break the line, nor can it enter the leakage switch.
4) If the earth leakage protector is used on L line, the working zero line must not be grounded repeatedly, and the PE line has repeated grounding, but it does not pass through the earth leakage protector, so the leakage protector can also be installed on the TN-S system power supply L line.
5) The TN-S power supply system is safe and reliable, suitable for low voltage power supply systems such as industrial and civil buildings. The TN-S power supply system must be used before the construction works begin.
TT power supply system
The TT method refers to a protective system that directly grounds the metal housing of an electrical device, which is called a protective earthing system, also called a TT system. The first symbol T indicates that the neutral point of the power system is directly grounded; the second symbol T indicates that the conductive part of the load device that is not exposed to the live body is directly connected to the ground, regardless of how the system is grounded. All grounding of the load in the TT system is called protective grounding. The characteristics of this power supply system are as follows.
1) When the metal shell of the electrical equipment is charged (the phase line touches the shell or the equipment insulation is damaged and leaks), the grounding protection can greatly reduce the risk of electric shock. However, low-voltage circuit breakers (automatic switches) do not necessarily trip, causing the earth-leakage voltage of the leakage device to be higher than the safe voltage, which is a dangerous voltage.
2) When the leakage current is relatively small, even a fuse may not be able to blow. Therefore, a leakage protector is also required for protection. Therefore, the TT system is difficult to popularize.
3) The grounding device of the TT system consumes a lot of steel, and it is difficult to recycle, time, and materials.
At present, some construction units use the TT system. When the construction unit borrows its power supply for temporary use of electricity, a special protection line is used to reduce the amount of steel used for the grounding device.
Separate the newly added special protection line PE line from the working zero line N, which is characterized by:
1 There is no electrical connection between the common grounding line and the working neutral line;
2 In the normal operation, the working zero line can have current, and the special protection line does not have current;
3 The TT system is suitable for places where ground protection is very scattered.
TN power supply system
TN mode power supply system This type of power supply system is a protection system that connects the metal housing of the electrical equipment with the working neutral wire. It is called the zero protection system and it is represented by TN. Its features are as follows.
1) Once the device is energized, the zero-crossing protection system can increase the leakage current to a short-circuit current. This current is 5.3 times larger than that of the TT system. Actually, it is a single-phase short-circuit fault and the fuse of the fuse will blow. The trip unit of the low-voltage circuit breaker will immediately trip and trip, making the faulty device powered off and safer.
2) The TN system saves material and man-hours and is widely used in many countries and countries in China. It shows that the TT system has many advantages. In TN mode power supply system, it is divided into TN-C and TN-S according to whether the protection zero line is separated from the working zero line.
In the TN system, the exposed conductive parts of all electrical equipment are connected to the protective line and connected to the ground point of the power supply. This ground point is usually the neutral point of the power distribution system. The power system of the TN system has one point that is directly grounded. The exposed electrically conductive part of the electrical device is connected to this point through a protective conductor. The TN system is usually a neutral-grounded three-phase grid system. Its characteristic is that the exposed conductive part of the electrical equipment is directly connected to the grounding point of the system. When a short circuit occurs, the short-circuit current is a closed loop formed by the metal wire. A metallic single-phase short circuit is formed, resulting in a sufficiently large short-circuit current to enable the protective device to act reliably to remove the fault. If the working neutral line (N) is repeatedly grounded, when the case is short-circuited, part of the current may be diverted to the repeated grounding point, which may cause the protection device to fail to operate reliably or to avoid the failure, thereby expanding the fault. In the TN system, that is, the three-phase five-wire system, the N-line and the PE-line are separately laid and insulated from each other, and the PE line is connected to the housing of the electrical device instead of the N-line. Therefore, the most important thing we care about is the potential of the PE wire, not the potential of the N wire, so repeated grounding in a TN-S system is not a repeated grounding of the N wire. If the PE line and N line are grounded together, because the PE line and N line are connected at the repeated grounding point, the line between the repeated grounding point and the working ground point of the distribution transformer has no difference between the PE line and the N line. The original line is the N line. The neutral current that is assumed is shared by the N line and the PE line, and part of the current is shunted through the repeated grounding point. Because it can be considered that there is no PE line on the front side of the repeated grounding point, only the PEN line consisting of the original PE line and N line in parallel, the advantages of the original TN-S system will be lost, so the PE line and N line cannot be Common grounding. Due to the above reasons, it is clearly stated in the relevant regulations that the neutral line (ie N line) should not be grounded repeatedly except for the neutral point of the power supply.
IT mode power supply system I indicates that the power supply side has no working ground, or is grounded at high impedance. The second letter T indicates that the load side electrical equipment is grounded.
The IT mode power supply system has high reliability and good security when the power supply distance is not long. It is generally used in places where no blackouts are permitted, or places where strict continuous power supply is required, such as electric power steelmaking, operating rooms in large hospitals, and underground mines. The power supply conditions in underground mines are relatively poor and the cables are susceptible to moisture. Using the IT-powered system, even if the neutral point of the power supply is not grounded, once the device is leaking, the relative ground leakage current is still small and will not damage the balance of the power supply voltage. Therefore, it is safer than the neutral grounding system of the power supply. However, if the power supply is used for a long distance, the distributed capacitance of the power supply line to the earth cannot be ignored. When a short-circuit fault or leakage of the load causes the device case to become live, the leakage current will form a path through the earth and the protection device will not necessarily act. This is dangerous. Only when the power supply distance is not too long is it safer. This type of power supply is rare on the construction site.
The meaning of the letters I, T, N, C, S
1) In the symbol of the power supply method stipulated by the International Electrotechnical Commission (IEC), the first letter represents the relationship between the power (power) system and the ground. For example, T indicates that the neutral point is directly grounded; I indicates that the power supply is isolated from the ground or that one point of the power supply is connected to the ground via a high impedance (for example, 1000 Ω;) (I is the first letter of the French word Isolation of the word “isolation”).
2) The second letter indicates the electrically conductive device exposed to the ground. For example, T means that the device shell is grounded. It has no direct relation with any other grounding point in the system. N means that the load is protected by zero.
3) The third letter indicates the combination of working zero and protective line. For example, C indicates that the working neutral line and the protection line are one, such as TN-C; S indicates that the working neutral line and the protection line are strictly separated, so the PE line is called a dedicated protection line, such as TN-S.